Methods for fabricating friction bearings of the above-mentioned kind are known from industrial practice. The friction bearing is thereby made of sintered bronze or sintered iron, respectively, and the pore volume of this sintered bearing is maximally 30%. Such a pore volume is necessary for assuring good lubricating properties of the bearing with sufficient consistency. In general, the porous bearing is infiltrated with oil under negative pressure or vacuum, respectively, with the result that the oil settles in the interconnected pore volume. In order to allow the lubricant to also settle in the interior area of the porous bearing, open pores are required.
In the first place, the disadvantage of said known friction bearings consists in a bad running performance at low circumferential speeds of the shaft and/or with large lateral forces exerted on the shaft.
If the circumferential speed of the shaft drops below a certain value with a constant lateral force, or if the lateral force exerted on the shaft exceeds a certain value at a constant circumferential speed, insufficient pressure is built up in the gap of the bearing between shaft and bearing to effect a hydrodynamic lubrication process where the shaft floats on a lubricating film.
At sufficiently high circumferential speeds or sufficiently low lateral forces, the shaft floats on said lubricant and the bearing friction is solely determined by the friction of the fluid.
At low circumferential speeds, a mixed friction prevails between the shaft and the bearing, where, besides the fluid friction, there is also direct contact between the shaft and the bearing. This range of mixed friction involves an increase in wear and tear and an increased development of heat due to the increased friction in the bearing.
Through a high lateral force exerted on the shaft, as occurs in particular with engine drives, the region of mixed friction is shifted towards higher running speeds; a direct contact between the shaft and the bearing already occurs at high speeds.
The above-mentioned friction bearing technology common in industrial practice has the disadvantage that it fails in the region of mixed friction, where the heat development caused by the large amount of dry friction is so high and where such high local temperatures in the bearing occur, that the lubricant decomposes and the porous bearing is subject to wear and tear. The shaft seizes the bearing.
It is, therefore, known from the EP 0 428 539 B1 to substitute the sintered bronze bearing with a metal-ceramics sinter mixture as, on one hand, the combination of material ceramics-ceramics for bearings and shafts has very low sliding friction coefficients and a high temperature resistance in the region of mixed friction and on the other hand, the metal has a good thermal conductivity.
It turned out, however, that with high lateral forces being exerted on a bearing made of a metal-ceramics sinter mixture and a ceramic shaft, a local decomposition of the oil occured in the area of mixed friction due to coking.
For this reason, ball bearings have so far been exclusively used in the region of mixed friction or with non-stationarily driven shafts. In small-sized electric motors and the drive shafts thereof, however, the construction size and the prices of the available small-sized ball bearings are in the range of those of an electric motor.